Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
Vertical and horizontal wind shears, yaw misalignment, and/or wind turbulence may act either collectively or individually for producing an asymmetric loading of the wind turbine. In particular, such an asymmetric loading may act across the wind turbine rotor. As a result, at least some elements of the wind turbine may be deformed. For example, the main shaft of the wind turbine may be bent (e.g., radially displaced) as a result of asymmetric rotor loading.
In order to mitigate the effect of the asymmetric loading of a wind turbine, conventional asymmetric load control (ALC) systems may use an array of sensors, such as proximity sensors, in the wind turbine to directly measure deformation of at least some elements of the wind turbine, such as a bending of the main shaft as described for example in U.S. Pat. No. 7,160,083 entitled Method and Apparatus for Wind Turbine Rotor Load Control. Further, a set of sensors for ALC may be provided in the yaw system to directly measure a yaw drive signal, such as described in U.S. Pat. Application No. 2012/0027589 entitled Method and Apparatus for Control of Asymmetric Loading of a Wind Turbine. In each instance, the ALC system uses signals generated by the ALC sensors for mitigating the effect of an asymmetric load of the rotor by, for example, controlling blade pitch and/or yaw alignment of the wind turbine. Accordingly, an ALC assembly may facilitate with reducing the effects of extreme loads and fatigue cycles acting on the wind turbine.
Current ALC assemblies, however, are only configured for detecting a limited number of deflections (i.e. main shaft deflection, fore-aft and side-side tower movement, and a yaw drive deflection), though new developments in tower technology have created a need for detecting additional loading parameters. For example, lattice tower structures, also known as space frame structures, utilize a highly engineered and optimized structure capable of handling unique static and dynamic loads that occur during wind turbine operation. Such tower structures, however, have generally lower torsional stiffness and frequencies. These characteristics greatly influence design costs and may make the tower more susceptible to twisting due to torsional loads that may frequently occur from asymmetric rotor loading. It would therefore be desirable to detect torsional movement and/or torsional loading in the tower before fatigue and extreme torsion occurs, thereby increasing the life of the tower.
Thus, an improved method and system for further reducing asymmetric loading and/or increasing the reliability of current ALC assemblies is desirable. Accordingly, a method and system for detecting and reducing torsional movement and/or torsional loading of a wind turbine tower would be advantageous.